This invention relates to methods for improving the mechanical strength of veneer sheets having lathe checks.
It is well known that veneer sheets produced by a veneer lathe, veneer slicer or the like have, on one or more surfaces thereof, a number of lathe checks, namely, tears or cracks brought about along the fibers of the sheets and at certain intervals depending upon the particular thickness of the sheet due to the bending of the sheets during the cutting operation thereof. Generally, the thinner the veneer sheet, the shallower its lathe checks, and the depth of lathe checks can be reduced as much as possible by selecting the most appropriate conditions for the cutting operation of the veneer sheets. However, it is very difficult to prevent the foregoing kinds of veneer sheets from having lathe checks. And, as shown in FIG. 6, such a veneer sheet 1 is usually curved to its tight side, that is, its side having no lathe checks.
Nevertheless, the inner openings or gaps of the veneer sheet 1 formed by its lathe checks 2 may be made smaller by bonding other boards, such as veneer sheets 30 with relatively small thicknesses and, hence, with no conspicuous lathe checks to both tight side (side having no lathe checks) and loose side (side having the lathe checks 2) of the veneer sheet 1 (after allowing the sheet 1 to dry or without doing so) (FIG. 10) so as to flatten the sheet 1. However, lathe checks are essentially tears, and the inner surfaces created thereby are very coarse. Therefore, the mere flattening of the sheet 1 may not completely or substantially close the openings and may not prevent at least some of the lathe checks from developing into noticeable gaps or openings in course of time.
Also, though on rare occasions, it may happen that very thick veneer sheets obtained from relatively slender logs and having lathe checks are curved to their loose sides (FIG. 7). Whether veneer sheets having lathe checks are curved to either side, such veneer sheets may be easily flattened, with a very small force, for use in manufacture of plywoods. That is, the mechanical strength of a veneer sheet having lathe checks is extremely low in its two opposite directions in the same plane as the sheet and substantially perpendicular to the directions of its fibers. Therefore such a sheet is usually not put to practical use, without improving its mechanical strength, except for such very special applications as manufacture of matchwood or small wooden boxes for packing food. Thus the practical value of such a veneer sheet as a single material is extremely small, and in most cases it is employed with another or other boards glued thereto.
However, as the usual lumber products of common materials show, wood itself is not necessarily so weak in the foregoing two opposite directions, but may have a sufficient mechanical strength suitable for practical use, depending upon their thicknesses. It is apparent that the very low mechanical strength of veneer sheets with lathe checks in its foregoing two opposite directions results from the presence of the lathe checks, and nothing but the lathe checks considerably reduces the practical value of the veneer sheets. Even if such a veneer sheet is bonded to other boards with relatively small thicknesses and, hence, with no conspicuous lathe checks, such a veneer sheet will certainly reduce the mechanical strength of such a product sooner or later. For example, in case of products where all or most of the veneer sheets bonded together are so arranged that their fibers extend in the same directions, their mechanical strength lowered by lathe checks might extremely restrict the range of use of such products. One example of such products is laminated veneer lumbers, which are expected to enjoy, as a substitute for the usual lumber products of common materials, a stronger and increasing demand in future.
For the purpose of improving the mechanical strength of veneer sheets having lathe checks, the inventor has carried out the following methods by using such veneer sheets having different moisture contents and divided into three groups at random:
(1) Filled glue into the lathe checks of each veneer sheet of the first group, flattened the sheet, and hardened the glue;
(2) Bonded other boards to both tight and loose sides of each veneer sheet of the second group with glue; and
(3) Filled glue into the lathe checks of each veneer sheet of the the third group, and bonded other boards to both tight and loose sides of the sheet with glue, and hardened the glue filled into the lathe checks.
None of the above-mentioned methods, however, has proved not to substantially increase the mechanical strength of the veneer sheets.
To be more exact, for example, if a veneer sheet having lathe checks and curved as shown in FIG. 6 is filled with glue in its lathe checks and, after the sheet is flattened, the glue is hardened, the lathe checks cannot be completely or substantially closed because they are essentially tears and the inner surfaces of the sheet created thereby are very coarse, having innumberable wood particles between them. Rather, not a few of the lathe checks may develop into noticeable gaps, sooner or later, in spite of the presence of the glue in the lathe checks. Also, if such a veneer sheet as shown in FIG. 6 is processed by the above-mentioned method (2) or (3), its lathe checks cannot be completely or substantially closed for the same reason. Therefore none of the above-mentioned methods (1) to (3) can be relied on considerably to increase the mechanical strength of the veneer sheet. Also, since the flattening of such a veneer sheet as shown in FIG. 7 makes its lathe checks larger, it is more difficult to increase the mechanical strength of such a veneer sheet by the same methods as above.
There is a tendency that the size of the openings formed by lathe checks is substantially directly proportional to their depths. Also, lathe checks tend to develop into noticeable gaps as the veneer sheets are dried. At any rate, with regard to the above-mentioned methods (1) and (3), it is not possible completely to fill up lathe checks with the usual amount of glue as determined from the cost of processing the sheets and the like.
The inventor has carried out the foregoing methods (1) and (3) by using not only the usual amount of glue, but also a larger amount of it. However, since, needless to say, a lathe check is narrower toward its inner portion, and innumberable wood particles exist inside it, it is very difficult to fill a larger amount of glue uniformly into a lathe check; rather, there is a tendency that more glue is supplied in the outer portion thereof. Also, needless to say, the use of an increased amount of glue increases the cost of processing the veneer sheets. Furthermore, it is well known that the use of a larger amount of glue for bonding things together may reduce the bonding strength of the glue itself. For example, when using an increased amount of ureic adhesive for the methods (1) and (3), which adhesive is desirable in its properties as well as reasonable in its cost, the inventor has encountered the problem that the adhesive produces foam where more of it is supplied, and cannot bond the inner surfaces of the veneer sheets formed by the lathe checks together, with its inherent strength. Thus the use of the increased amount of adhesive could not increase the mechanical strength of the sheets proportionally.
Thus the inventor has recognized that none of the foregoing methods is a satisfactory one for improving the mechanical strength of veneer sheets having lathe checks, but found that it may be substantially increased by improving the foregoing methods as mentioned hereinafter.